1. Field of the Invention
The present invention relates generally to digital communication systems, and more particularly to a methods and apparatus for phase reference tracking of digital phase modulated signals in the receiver.
2. Background
Digital phase modulation is one of the popular digital modulations due to its simplicity and robustness. Source information is transmitted by selecting phases of the signal according to the information bits. Continuous phase modulation (CPM), Phase-shift keying (PSK) and differential phase-shift keying (DPSK) are examples of digital phase modulation.
In the receiver, it is necessary to detect an accurate phase reference for decoding the transmitted information bits. Otherwise, phase reference errors may cause significant performance degradation. For differentially encoded digital phase modulations such as DPSK, DQPSK and D8PSK, a phase reference can be derived from the previous symbol to facilitate the demodulation. For simple receivers, DPSK signals may be differentially decoded. That means, previous phase is used as a reference for the current symbol. However, since this reference is noisy, the performance can degrade up to 3 dB, compared to the performance with perfect phase reference. Phase reference tracking is also useful for DPSK signals.
To facilitate such a phase reference estimate, a training sequence is often transmitted at the beginning of a data packet. The phase reference may be easily estimated with the training sequence known at the receiver, but the throughput may be slightly decreased as the training sequence does not contain source information. Moreover, the phase reference may be time-varying due to the imperfect oscillators at the transmitter (TX) or the receiver (RX). In this case, phase reference tracking will be necessary for the receiver to maintain best performance while receiving information bits. Phase references may be heavily time-varying due to the mismatching between TX and RX oscillators. This mismatching is so-called frequency offset (FO). Moreover frequency drift may cause difficulty in tracking accurate phase reference. By estimating and/or tracking this FO, phase reference may be kept accurate.
For this phase reference tracking, multiple symbol detection [1] based on maximum likelihood sequence detection (MLSD) was proposed, but its complexity exponentially increases with the number of observation symbols. Furthermore, U.S. Pat. No. 7,245,672 issued to Smit et al., entitled “Method and apparatus for phase-domain semi-coherent demodulation” disclosed that a first-order IIR filter in phase-domain, of which complexity further decreases due to the phase operations instead of the complex signal operations as shown in FIG. 3, However, this first order phase tracking is not sufficient to handle heavy phase variations. For example, Bluetooth spec [2] allows frequency to drift up to 400 Hz/μs. Also, double errors (decoded symbol error propagation) are un-avoidable in this invention due to the proposed differential decoding.
According to above problems, the related field needs a simple and robust phase tracking method, where the phase reference is tracked in phase-domain with a high order digital phase-locked loop, for general phase-modulated signals. Also, related field suggests a better way to track phase for differentially encoded digital phase modulations, such as DBPSK, (π/4) DQPSK and DBPSK modulated signals.